Amplifier



R. M. STEERE Aug. 6, 1940.

AMPLIFIER Filed Jan. 31, 1938 2 Sheets-Sheet l 3nventor Richard, M Ste Cl ttorneg Aug. 6, 1940. 2.210.387.

R. M. STEERE AMPLIFIER Filed Jan. 31', 1938 2 Sheets-Sheet 2 CONTROL GRID B/flnS' V0576 NEGH'I'IVE' ZSnventor Hich/ard MJteere HAL Q Patented Aug. 6, 1946 LIFIER Richard M. Steere, Haddoniicld,

to Radio Corporation of America,

of Delaware N. J., assignor a corporation Application January 31, 1938, Serial No. 187,940-

10 Claims.

The present invention relates to amplifiers for radio signalling systems and the like, embodying tuned high frequency signal conveying circuits: in 7 connection with pacitance of eiectric and the like.

When such devices, in connection with tuned high frequency circuits, are subjected to gain oontrolling potentials on the input or control grid, the input capacitance may vary sufiiciently to cause a change in the tuning of the circuits connected therewith beyond a desired or permissible limit, and itis an object of the present invention to provide means for preventing such changes in the input capacitance discharge device in connection with a tuned circuit from changing the tuning thereof.

More particularly, it is an object of the present invention to provide, in connection with a tuned signalling circuit, electric dischargev ampl-ifier means wherein changes: in input capacitance with changes in biasing potential maybe made substantially equal in magnitude and opposite in direction of change, with changes in biasing potential. With this arrangement, automatic volume control potentials may be applied at an electric discharge amplifier, mixer tube or detector in connection with a tuned signalling circuit, without causing changes in capacitance across and variations in tuning of the circuit with variations in the magnitude of the automatic volume control potential.

It may also be considered as a specific object of the present invention, to provide means for i compensating for the shift of the resonance frequency of tuned signalling circuits in electric discharge amplifiers tending toresult from change in input capacitance an electric discharge dedischarge amplifier devices of vice with varying grid bias potential, more per- I ticularly when such circuits operate at high frequencies or high intermediate frequencies and have a high L/C'. ratio,

It is a still furtherobject of the present invention, to provide in a single electric discharge device, amplifying, mixing or detecting and compensating means for resonant'frequency shift of a tuned signalling circuit in connection with an electric discharge device because of input capacitance variation with varying grid voltage,

The invention will, however, be better understood from the following description, when considered in connection with the accompanying drawings, and its scope will be pointed out in the appended claims. I

In the drawings,

the input or grid-to-cathode ca:

of an electric Figure l is a schematic circuit diagram of a portion of an electric signalling system, such as an intermediate frequency amplifier of a superheterodyne receiver, embodying the invention,

Figure 2 is a curve showing input capacitance variation of a standard amplifier tube withvariation in control grid bias, and

Figure 3 is a schematic circuit diagram, showing a modification oi the circuit of Fig; 1 also embodying the invention.

Referring to Fig, 1, ii is an electric discharge device representing an amplifier, signal mixer or detector having input or control grid 6 and a cathode 1 connected with a. tuned signal conveying circuit 3, having a high L/C ratio and comprising a secondary 9 of anintermediate ire quency amplifier input transformer l0 and. a shunt tuning capacitor Thehigh potential terminal |2 of the tuned input circuit is connected with the grid 6, while the low potential terminal M of the tuned input circuit is connected through a by-pass capacitor IE to' ground indicated at 36, which may be the chassis of the apparatus with which the amplifier 5 is associated. j 25 The cathode is by-passed to ground by a capacitor indicated at H, across a' self-bias resistor N3, the input capacitance of the tube 5 between the control or input grid 6 and cathode "I is eiiectively in shunt across the tuned signal circuit 8. A variable biasing potential is applied to the grid 6 from an automatic volume control lead' indicated at 20, which in turn receives a negative variable biasing potential responsive to variations in the amplitude of a received carrier wave from a signal rectifier or second detector indicated at 2| through the rectifier output resistor indicated at 22. The signal rectifier or second detector 2| is coupled to the amplifier 5 to receive amplified signals therefrom, such as a modulated carrier wave, through suitable intermediate amplifying apparatus represented by the rectangle 23, which is coupled to the amplifier 5 through a tuned intermediate frequency interstage transtformer 45 indicated at 24 and to the detector 2| through a tuned intermediate frequency output transformer indicated at 25. Automatic volume control potentials for the'amplifier 23 are provided through, a filter resistor lead 20, and a similar filter resistor 21 is provided for the amplifier 5. Asiswell understood, the amplifier 23 may include any suitablefnumber of electric discharge amplifier stages. 5

The second detector or rectifier 2|, is preier- 26 connected with the 50' ably of the diode type having an anode'28 and a cathode 29 connected across the tuned secondary 33 of the intermediate frequency output transformer 25 through the output resistor 22 and a similar output resistor 3| in series therewith between the cathode and the low potential terminal of the secondary 39. The terminal 32 between the resistors 22 and 3| is connected to the chassis or ground, as indicated at 33. Both resistors are suitably by-passed to eliminate the carrier wave, as indicated by the by-pass capacitors 34. 1

As indicated in the drawings, the cathode terminal 35 of the resistor 3| is made positive, in response to signal flow, while the terminal 36 of the resistor 22 becomes negative, with respect to the grounded terminal 32. If the resistors 22 and 3| are made equal in value, the terminal 35 will become positive at the same rate that the terminal 36 becomes negative and as the signal strength of the carrier wave amplitude increases.

The automatic volume control lead 29 is connected to the terminal 33 to receive an increasing negative biasing potential in response to an increase in signal strength or carrier wave amplitude with respect to chassis or ground 33, thereby to decrease the gain in the amplifiers 5 and 23 as is well understood. Also the modulation component or audio frequency signal may be derived "from the output resistor 22 of the detector 2| through an output coupling capacitor 31 and an audio frequency output lead 38.

It should be understood that the audio frequency amplifying portion of the system, as well as the signal input portions of the receiving system connected with the primary 39 of the input transformer H] are not shown, for the purpose of simplifying the drawings and for the further reason that the present invention is not concerned with such portions of the receiving system.

The input capacitance of the tube 5 varies with variation in the biasing potential provided by the automatic volume control lead 29 in response to signals, and decreases from a fixed value determined by the biasing potential derived from the fixed source l8, which is in series with the variable source 22 between the grid 6 and the cathode I and tends to change the tuning of the input circuit 8 of the amplifier. The variation in input capacitance of a typical screen grid pentode amplifier tube, such as the tube 5, and represented by a type known commercially as an RCA 6K7 Radiotron, is shown by the curve 49 in Fig. 2, to which attention is now directed along with Fig. 1. g

It will be seen from a consideration of Fig. 2, that the input capacitance decreases rapidly over a short range of biasing potential change, which, for the tube under consideration, may be in the range of from three to ten volts negative, and that this change is substantially linear.

If the control grid of an auxiliary or compensating electric discharge device or tube 39, having a similar relationship between control grid bias and input capacitance to that of the tube 5, is coupled to the tuned high frequency circuit 8, to place its input capacitance across the circuit in parallel with that of the tube 5, by applying a suitable potential to the control grid indicated at 40, the change in input capacitance of the tube 5 may be compensated.

In the practice, as indicated in the drawings, the control grid 4| of the auxiliary or compensating tube 39 may be coupled through a suitable coupling capacitor, indicated at 42 to the high potential terminal I 2 of the input circuit 8 and the cathode indicated at 43 may be connected through a suitable self-bias resistor or other fixed bias potential source 44 to ground 45, thereby placing the input capacitance of the tube 39 across the circuit 8. The tube 39 is preferably the same type of tube as that shown at 5, although any suitable tube may be used at 39 to provide a similar relationship to tube 5 between the input capacitance and grid bias variation.

Assuming that the tubes have similar characteristics, as shown in Fig. 2, the tube 5 is normally biased to a point A on the curve 40 which corresponds to substantially three volts negative bias, while the compensating grid 4| is biased normally to the point 13 corresponding to substantially ten volts negative bias. The initial or fixed biasing potentials referred to are supplied by the fixed sources l8 and 44, respectively.

If the signal rectifier output resistors 22 and 3| are made substantially equal and the control grid 4| of the compensating tube 39 is connected through a grid resistor 46 and compensating bias supply lead 41 to the positive terminal 35 of the diode output circuit, the bias potentials with respect to the ground terminal 32 from the rectifier or diode output circuit, in response to a received signal, will become increasingly positive on the grid 4| and increasingly negative on the grid 6 at the same rate, so that the biasing potential indicated at A on curve 40 will increase in a negative direction and that at B will increase in a positive direction, thereby giving substantially no input capacitance change across the circuit 8.

If the two tubes are of the same construction, the. changes in capacitance may be made substantially equal in magnitude and opposite in direction by applying to the two tubes biasing potentials which vary at equal rates with variations in the strength of an applied signal, and opposite in the direction of the polarity change.

If the tubes 5 and 39 are of different types, the resistors 22 and 3| may be proportioned in accordance with the characteristics of the two tubes so that the input capacitance changes may be made substantially equal and opposite in response to a change in signal strength and a corresponding change in the A. V. C. potential.

With tubes of differing construction for the amplifier and compensating portions of the system, the input capacitance changes may further be adjusted whereby they vary at substantially equal rates and opposite in direction in response to a varying biasing potential, by adjustment of the plate and screen grid voltages applied to the tube 39. In the present example, the screen grid 41 of the tube 39 is connected through a variable contact 48 with a potential source 49 and the anode 50 is connected through an output resistor 5| with a second variable contact 52 in connection with the source 49, whereby the anode and,

screen grid voltages may be adjusted to provide an input capacitance for the tube 39 to substantially counteract the input capacitance shift of the tube 5.

From the foregoing description, it will be seen that, in connection with tuned high frequency circuits, particularly circuits having a high L/C ratio, a variable biasing potential may be applied to the control grids of electric discharge tub-es connected with such circuits without permitting the input capacitance changes resulting therefrom to change the resonant frequency of said circuits. In accordance with the invention, this is prevented by causing the mean input capacitance across the tuned circuit to be maintained constant or substantially constant between limits which do not appreciably afiect the tuning,

two grids which vary in predetermined relation to each other in magnitude and polarity.

A specific. embodiment comprises a tuned high frequency circuit having the control grid of an electricdischarge device connected therewith and automatic volume control means for said device connected with said grid to apply thereto a variable biasing potential. When this potential variation causes the input capacitance to vary between limits affecting the resonant frequency of the circuit, an additional'grid connection and a second or additional source of A. V. C. potential which varies with the first A. V. C. potentialin an opposite sense is provided. The second source is connected with the second grid and is adjusted for operation to cause the input capacitance changes to be substantially equaland opposite in response to varying signal strength and to maintain the resonant frequency of the circuit substantially constant.

If desired, the two control grids may be in cluded in one electric discharge tube orenvelope, as shown in the modified circuit of Fig. 3, wherein the two tubes 39 and 5 are combined in one tube and operated in connection with the same circuit element as in Fig. l, which bear the same reference numerals as in Fig'. 1.

Referring to- Fig. 3, the tuned input circuit 8 is connected with an amplifier control grid 6 in a common envelope 55, with the compensating control grid 4!, the latter being associated with the cathode i3 and the former being associated with the cathode 7. Each of the cathodes is provided with a separate connection to ground t5 and 16 through their respective grid bias resistors 44 and it). The amplifier output is taken through the output transformer 24 and the positive and negative A. V. C. potentials are applied to the grids 4i and 6 through the leads Al and 26 from the A. V. C. rectifier output resistors 2i and 22. The grid M is operated normally at the point B on the curve All of Fig. 2 and the grid 5 is operated normally at the point A on the curve at of Fig. 2.

In response to received signals, the fixed'negative bias potential on the grid dl, as provided by the source 44, is reduced while the'fixed negative bias potential on the grid 6, as provided by the source 68, is increased.

In the'present example, the compensating tube portion of the device 55 is a triode and the anode potential may be adjusted, together with the output resistors 2| and 22, to provide equal and opposite input capacitance change with variation in potential from the A. V. C. rectifier: Other wise, the operation of the circuit of Fig. 3 is the same as that of Fig. l.

The advantage of the circuit of Fig. 3 lies in the fact that a single tube is required for amplification and compensation, and these functions may be combined in one envelope and require relatively short lead connections to the various elements for both amplification and compensation. I claim as my invention: 1. The combination with a tuned high fre quency signal circuit, of an electric discharge device having a control grid and a cathode connected with said circuit providing a predetermined input cap'acitarice across said circuit, means for applying a variable biasing potential to said control grid which changes said input capacitance, electric discharge means providing a second control grid and cathode input capacitance across said tuned circuit, and means for causing said last named input capacitance to vary simultaneously with the first named inputcapacitance in an opposite sense and at a predetermined rate with respect thereto.

Z. The combination with a tuned high frequency signal circuit, of an electric discharge device having a control grid and a cathode connected with said circuit providing a predetr input capacitance across said circuit,

means for applying a variable biasing potential variablebiasing means" for receiving therefrom a biasing potential which varies at a predetermined rate in magnitude and polarity, with re spect to the potential applied to said first named grid.

3. lihe combination with a tuned high frequency signal conveying circuit having a high L/C ratio, of electric discharge means providing a predetermined mean input capacitance across said circuit and including two control grids connected with said circuit, means for applying predetermined fixed biasing potentials of differing magnitude to said grids whereby the input capacitance of one grid is reduced and the input capacitance of the other grid is increased with respect to said mean input capacitance, and variable biasing means connected with said grids for causing the input capacitance of said grids to vary at a predetermined rate and in an opposite sense, one with respect to the other, whereby the mean input capacitance across said circuit remains substantially constant.

4. The combination with a tuned high frequency signal conveying circuit, of electric distion with a tuned high frequency circuit, of an electric discharge amplifier device having a control grid connected with said circuit, means for subjecting said grid to a variable gain controlling potential, means for compensating the effect of a change in the input capacitance of said tube comprising a second control grid coupled to said circuit and connected with said first named means to receive therefrom a gain controlling potential of opposite polarity which varies in magnitude with the first named potential, said first named means including signal rectifier circuit elements for maintaining a predetermined change of said potentials and the input capacitance changes provided by said grids substantially equal and opposite.

6. The combination with a tuned high frequency signal conveying circuit, of electric discharge means providing a predetermined mean input capacitance across said circuit and including two control grids connected with said circuits, means for applying predetermined fixed biasing potentials of differing magnitude to said grids whereby the input capacitance of one grid is reduced and the input capacitance of the other grid is increased with respect to said mean input capacitance, and means for causing the said input capacitance of said grids to vary at a predetermined rate and in an opposite sense, one with respect to the other, whereby said mean input capacitance across said circuit remains substantially constant, said last named means comprising a source of negative biasing potential for one of said grids and a source of positive bias potential for the other of said grids conjointly variable to cause said potentials to increase and decrease simultaneously at predetermined rates, one with respect to the other.

'7. In a signaling system, the combination with a tuned high frequency signal conveying circuit of an electric discharge device having a first control grid connected with said circuit providing a predetermined input capacitance for said circuit, a second control grid providing a predetermined compensating input capacitance for said circuit, and means providing a change in the input capacitance provided by one grid substantially equal in magnitude and opposite in direction to changes in the input capacitance provided by the other of said grids in response to variations in grid bias applied to said grids, said last named means comprising a grid bias potential supply circuit connected to each of said grids and a variable potential source connected with said circuits and providing potentials of opposite po larity on said grid bias potential supply circuits and a predetermined rate of change of magnitude for said potentials, one with respect to the other.

8. In an electric discharge amplifier, the combination with a tuned high frequency circuit, of an electric discharge amplifier device having a control grid and a cathode connection providing input capacitance across said circuit, means for adjusting said input capacitance to a predetermined value, means including a second control grid and cathode connection for said circuit and relation between the magnitude and rate of a second input capacitance across said circuit, means for adjusting said last named input capacitance to a predetermined value difiering from the first named input capacitance, means for applying variable biasing potentials to said control grids to vary the gain of said amplifier and for causing said input capacitances to vary at predetermined rates and in an opposite sense, one with respect to the other.

9. The combination with a tuned high frequency signal conveying circuit, of electric discharge means providing a predetermined mean input capacitance across said circuit and including two control grids connected with said circuits, means for applying predetermined fixed biasing potentials of differing magnitude to said grids whereby the input capacitance of one grid is reduced and the input capacitance of the other grid is increased with respect to said mean input capacitance, and means for applying additional biasing potentials to said control grids which vary at a predetermined rate and in opposite sense, one with respect to the other, said last named means comprising a signal rectifier for receiving an applied signal, an output resistor for said rectifier having a positive terminal connected with one of said control grids, a negative terminal connected with the other of said control grids and a tap intermediate between said terminals providing a cathode connection.

10. In an electric signalling system, the combination with a tuned signal transmitting circuit, of an electric discharge device having a control grid connected with said circuit and providing input capacitance across said tuned circuit variable with variations in grid bias, means for applying to said control grid a variable biasing potential, and electric discharge means for preventing the tuning of said circuit from varying in response to variations in said input capacitance, said last-named means including a second control grid coupled to said input circuit to provide a second variable input capacitance across said circuit, and means jointly operable with said first-named grid bias means for applying to said second grid a biasing potential which varies in an opposite sense from the variations in the biasing potential on the firstnamed grid and so proportioned with respect thereto that said input capacitance variations on said tuned circuit may be made substantially equal and opposite.

RICHARD M. STEERE. 

